1. Field of the Invention
The present invention relates to secondary (rechargeable) alkali-ion batteries. More specifically, the invention relates to materials for use as electrodes for an alkali-ion battery. The invention provides transition-metal compounds having the ordered olivine, the modified olivine or the rhombohedral NASICON structure and containing the polyanion (PO4)3xe2x88x92 as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.
2. Description of the Related Art
Present-day lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li+ ions to the Li+-ion electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li+ ions are inserted reversibly from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible. On charge, removal of electrons from the cathode by an external field releases Li+ ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li+ ions back into the anode to restore it to its original composition.
Present-day rechargeable lithium-ion batteries use a coke material into which lithium is inserted reversibly as the anode and a layered or framework transition-metal oxide is used as the cathode host material (Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/or Ni are expensive and may degrade due to the incorporation of unwanted species from the electrolyte. Oxides such as Li1xc2x1x[Mn2]O4, which has the [M2]O4 spinel framework, provide strong bonding in three dimensions and an interconnected interstitial space for lithium insertion. However, the small size of the O2xe2x88x92 ion restricts the free volume available to the Li+ ions, which limits the power capability of the electrodes. Although substitution of a larger S2xe2x88x92 ion for the O2xe2x88x92 ion increases the free volume available to the Li+ ions, it also reduces the output voltage of an elementary cell.
A host material that will provide a larger free volume for Li+-ion motion in the interstitial space would allow realization of a higher lithium-ion conductivity "sgr"Li, and hence higher power densities. An oxide is needed for output voltage, and hence higher energy density. An inexpensive, non-polluting transition-metal atom would make the battery environmentally benign.
The present invention meets these goals more adequately than previously known secondary battery cathode materials by providing oxides containing larger tetrahedral oxide polyanions forming 3D framework host structures with octahedral-site transition-metal oxidant cations, such as iron, that are environmentally benign.
The present invention provides electrode material for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte. The cell may additionally include an electrode separator. As used herein, xe2x80x9celectrochemical cellxe2x80x9d refers not only to the building block, or internal portion, of a battery but is also meant to refer to a battery in general. Although either the cathode or the anode may comprise the material of the invention, the material will preferably be useful in the cathode.
Generally, in one aspect, the invention provides an ordered olivine compound having the general formula LiMPO4, where M is at least one first row transition-metal cation. The alkali ion Li+ may be inserted/extracted reversibly from/to the electrolyte of the battery to/from the interstitial space of the host MPO4 framework of the ordered-olivine structure as the transition-metal M cation (or combination of cations) is reduced/oxidized by charge-compensating electrons supplied/removed by the external circuit of the battery in, for a cathode material, a discharge/charge cycle. In particular, M will preferably be Mn, Fe, Co, Ti, Ni or a combination thereof. Examples of combinations of the transition-metals for use as the substituent M include, but are not limited to, Fe1xe2x88x92xMnx, and Fe1xe2x88x92xTix, where 0 less than x less than 1.
Preferred formulas for the ordered olivine electrode compounds of the invention include, but are not limited to LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4, and mixed transition-metal compounds such as Li1xe2x88x922xFe1xe2x88x92xTixPO4 or LiFe1xe2x88x92xMnxPO4, where 0 less than x less than 1. However, it will be understood by one of skill in the art that other compounds having the general formula LiMPO4 and an ordered olivine structure are included within the scope of the invention.
The electrode materials of the general formula LiMPO4 described herein typically have an ordered olivine structure having a plurality of planes defined by zigzag chains and linear chains, where the M atoms occupy the zigzag chains of octahedra and the Li atoms occupy the linear chains of alternate planes of octahedral sites.
The present invention additionally provides electrode material for a rechargeable electrochemical cell including an anode, a catrode and an electrolyte where the material has a modified olivine structure. The pristine olivine structure of LiMPO4 may be modified either on the anionic site or on the cationic site to provide an alternative lithium insertion-type. It is also envisioned that the pristine olivine structure may be modified on both the anionic and the cationic sites. Preferably, the structure is modified by aliovalent or isocharge substitutions to provide better lithium ion diffusitivity and electronic conductivity.
In general, xe2x80x9cisocharge substitutionsxe2x80x9d refers to substitution of one element on a given crystallographic site with an element having a similar charge. For example, Mg2+ is considered similarly isocharge with Fe2+ and V5+ is similarly isocharge with P5+. Likewise, PO43 tetrahedra can be substituted with VO43 tetrahedra. xe2x80x9cAliovalent substitutionxe2x80x9d refers to substitution of one element on a given crystallographic site with an element of a different valence or charge. One example of an aliovalent substitution would be Cr3+ or Ti4+ on an Fe2+ site. Another example would be Li+ on a Fe2+ site. These cathode materials will generally have an olivine structure based on iron or manganese derivatives whose general formula is:
Lix+yM1xe2x88x92(y+d+t+q+r)DdTtQqRr[PO4]1xe2x88x92(p+s+v)[SO4]p[SiO4]s[VO4]v
where
M may be Fe2+ or Mn2+ or mixtures thereof;
D may be a metal in the +2 oxidation state, preferably Mg2+, Ni2+, Co2+, Zn2+, Cu2+, or Ti2+;
T may be a metal in the +3 oxidation state, preferably Al3+, Ti3+, Cr3+, Fe3+, Mn3+, Ga3+, Zn3+, or V3+;
Q may be a metal in the +4 oxidation state, preferably Ti4+, Ge4+, Sn4+, or V4+;
R may be a metal in the +5 oxidation state, preferably V5+, Nb5+, or Ta5+;
In this preferred embodiment, M, D, T, Q and R reside in octahedral sites. The additional coefficients may be defined as follows: x represents the degree of intercalation during operation of the electrode material; y represents the fraction of lithium ions on the initial Fe2+ sites; d represents the fraction of divalent ions (noted as D) on the initial Fe2+ sites; t represents the fraction of trivalent ions (noted as T) on the initial Fe2+ sites; q represents the fraction of tetravalent ions (noted as Q) on the initial Fe2+ sites; r represents the fraction of pentavalent ions (noted as R) on the initial Fe2+ sites; p represents the fraction of hexavalent sulfur (as discrete SO42xe2x88x92 tetrahedra) on the initial p5+ sites; s represents the fraction of tetravalent silicon (as discrete SiO42xe2x88x92 tetrahedra) on the initial P5+ sites; and v represents the fraction of pentavalent vanadium ions on the initial P5+ sites.
The conditions for site occupancy and electroneutrality imply the following:
0xe2x89xa6xxe2x89xa61;
y+d+t+q+rxe2x89xa61;
P+s+v less than 1; and
3+sxe2x88x92p=xxe2x88x92y+t+2q+3r.
x, y, d, t, q, r, p, s, and v may be between 0 (zero) and 1 (one), with at least one of y, d, t, q, r, p, s, or v differing from 0. In a preferred embodiment y, d, t, q, r, and v may vary between 0 (zero) and 0.2 ({fraction (2/10)}) and r and s may vary between 0 (zero) and 0.5 (xc2xd).
The invention further provides an electrochemical cell or battery having at least one positive and one negative electrode. At least one positive electrode in this embodiment contains the Lix+yM1xe2x88x92(y+d+t+q+r)DdTtQqRr[PO4]1xe2x88x92(p+s+v)[SO4]p[SiO4]s[VO4]v material described above. Further, at least one negative electrode contains a source of lithium ion at a high chemical activity. The phrase xe2x80x9chigh chemical activityxe2x80x9d is generally understood in the art to refer to an electrode whose mean voltage during operation is not more positive than 2 volts versus the Li+/Lio couple.
Preferably, at least one negative electrode will contain metallic lithium, a lithium alloy, a lithium-carbon intercalation compound, a lithium-transition metal mixed nitride of antifluorite, or a lithium-titanium spinel Li1+x+zTi2xe2x88x92xO4, where 0xe2x89xa6xxe2x89xa61/3 and 0xe2x89xa6zxe2x89xa61-2x. It will be understood by those of skill in the art that the term xe2x80x9caxe2x80x9d used before a compound encompasses structures containing more than one of that type of compound. For example, xe2x80x9ca lithium-transition metal mixed nitride of antifluoritexe2x80x9d encompasses mixtures of more than one of this type of compound and xe2x80x9ca lithium-titanium spinelxe2x80x9d encompasses solid solutions and/or mixtures of more than one of this type of compound with other spinels.
Alternatively, the electrochemical cell of the invention may contain an intercalation material with fast diffusion kinetics in the positive electrode containing the Lix+yM1xe2x88x92(y+d+t+q+r)DdTtQqRr[PO4]1xe2x88x92(p+s+v)[SO4]p[SiO4]s[VO4]v material describe. The phrase xe2x80x9cfast diffusion kineticsxe2x80x9d is generally understood in the art as referring to a material able to sustain a specific current of at least 10 mA per gram of material with more than 80% utilization of the capacity at the temperature of operation. Preferably, the intercalation material with fast diffusion kinetics may be a lamellar dichalcognenide, a vanadium oxide VOx where 2.1xe2x89xa6xxe2x89xa62.5, or a NASICON-related material such as Li3Fe2(PO4)3 or Li3xe2x88x92xFe2xe2x88x92xTix(PO4)3 where x represents the degree of substitution of Fe3+ by Ti4+.
In other preferred aspects, the electrochemical cell of the invention will include a conductive additive in at least one positive electrode. The conductive additive may preferably be carbon.
In other aspects, it is envisioned that the electrochemical cell of the invention includes at least one positive electrode containing the Lix+yM1xe2x88x92(y+d+t+q+r)DdTtQqRr[PO4]1xe2x88x92(p+s+v)[SO4]p[SiO4]S[VO4]v material described above and a polymeric binder. In certain preferred aspects, this positive electrode may additionally include a conductive additive, such as carbon.
Preferably, the polymeric binder may be a homopolymer or copolymer of tetrafluoroethylene, an ethylene-propylene-diene terpolymer, a polyether, a polyester, a methylmethacrylate-based polymer, an acrylonitrile-based polymer, or a vinylidene fluoride-based polymer. It is contemplated that the polymeric binder for use in conjunction with the present invention may be crosslinked, but those of skill in the art will appreciate that cross-linkage is not essential. The term xe2x80x9ccrosslinkedxe2x80x9d refers to the presence of physical or chemical bonds between the polymer chains. Generally, those skilled in the art measure crosslinkage in terms of the number of crosslinks per cubic centimeter. The polymeric binder for use in conjunction with the present invention will preferably have a cross-linkage of between 1018 and 1020 inter-chain bonds per cubic centimeter.
Alternatively, the polymeric binder may possess ionic conductivity. Ionic conductivity is generally understood in the art to be the ability to carry a current due to the motion of ions. Preferred values of ionic conductivity are between about 10xe2x88x927 and about 10xe2x88x922 (Scmxe2x88x921). In certain embodiments, the polymeric binder may be swollen by an aprotic solvent which contain a salt, the cation of which is at least in part Li+. The aprotic solvent may preferably be ethylene carbonate, propylene carbonate, dimethylcarbonate, diethylcarbonate, methyl-ethylcarbonate, xcex3-butyrolactone, a tetraalkylsulfarniide, or a dialkylether of a mono-, di-, tri-, tetra- or higher oligo-ethylene glycol of molecular weight lower or equal to 2000 and mixtures thereof.
The invention further provides a variable optical transmission device which is constructed from transparent semi-conductor coated glass or plastic and includes two electrodes separated by a solid or gel electrolyte. In this embodiment, at least one electrode contains the Lix+yM1xe2x88x92(y+d+t+q+r)DdTtQqRr[PO4]1xe2x88x92(p+s+v)[SO4]p[SiO4]s[VO4]v material as described above. Preferably, the Lix+yM1xe2x88x92(y+d+t+q+r)DdTtQqRr[PO4]1xe2x88x92(p+s+v)[SO4]p[SiO4]s[VO4]v compound of the invention is layed on a transparent semiconductor coated glass or plastic in a thin film. It is preferred that the semi-conductor be coated onto the glass or plastic at a thickness of between 200 and 104 xc3x85ngstrxc3x6ms (xc3x85) or between 20 and 104 nanometers (nm). The material of the invention may be placed onto the glass or plastic, for example, using a vacuum deposition technique, by sputtering, or from a sol-gel precursor. The techniques for placing the compound of the invention onto the glass or plastic are well known to those skilled in the art. Preferred techniques include sputtering, chemical vapor deposition (CVD) from organometallic precursors like metal hexafluoroacetylacetonates and organic phosphates or silicates, sol-gel from hydrolysis-condensation of metal alkoxides in water-organic solutions in the presence of phosphoric acid and organosiloxanes.
It is preferred that the glass for use in conjunction with the present invention be conventional optical quality glazing. Preferred plastics include high transparency, high mechanical strength material like polyethylene terephatalate (Mylar(copyright)). The transparent semi-conductor is contemplated to be virtually any transparent semi-conductor but is preferably antimony- or fluorine-doped tin oxide, tin- or fluorine-doped indium oxide, or non-stoichiometric zinc oxide.
In another aspect, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials comprise a rhombohedral NASICON material having the formula YxM2(PO4)3, where 0xe2x89xa6xxe2x89xa65. Preferably, the compounds of the invention will be useful as the cathode of a rechargeable electrochemical cell. The alkali ion Y may be inserted from the electrolyte of the battery to the interstitial space of the rhombohedral M2(XO4)3 NASICON host framework as the transition-metal M cation (or combination of cations) is reduced by charge-compensating electrons supplied by the external circuit of the battery during discharge with the reverse process occurring during charge of the battery. While it is contemplated that the materials of the invention may consist of either a single rhombohedral phase or two phases, e.g. orthorhombic and monoclinic, the materials are preferably single-phase rhombohedral NASICON compounds. Generally, M will be at least one first-row transition-metal cation and Y will be Li or Na. In preferred compounds, M will be Fe, V, Mn, or Ti and Y will be Li.
Redox energies of the host M cations can be varied by a suitable choice of the XO4 polyanion, where X is taken from Si, P, As, or S and the structure may contain a combination of such polyanions. Tuning of the redox energies allows optimization of the battery voltage with respect to the electrolyte used in the battery. The invention replaces the oxide ion O2xe2x88x92 of conventional cathode materials by a polyanion (XO4)mxe2x88x92 to take advantage of (1) the larger size of the polyanion, which can enlarge the free volume of the host interstitial space available to the alkali ions, and (2) the covalent Xxe2x80x94O bonding, which stabilizes the redox energies of the M cations with Mxe2x80x94Oxe2x80x94X bonding so as to create acceptable open-circuit voltages Voc with environmentally benign Fe3+/Fe2+ and/or Ti4+/Ti3+ or V4+/V3+ redox couples.
Preferred formulas for the rhombohedral NASICON electrode compounds of the invention include, but are not limited to those having the formula Li3+xFe2(PO4)3, Li2+xFeTi(PO4)3, LixTiNb(PO4)3, and Li1+xFeNb(PO4)3, where 0 less than x less than 2. It will be understood by one of skill in the art that Na may be substituted for Li in any of the above compounds to provide cathode materials for a Na ion rechargeable battery. For example, one may employ Na3+xFe2(PO4)3, Na2+xFeTi(PO4)3, NaxTiNb(PO4)3 or Na1+xFeNb(PO4)3, where 0 less than x less than 2, in a Na ion rechargeable battery. In this aspect, Na+ is the working ion and the anode and electrolyte comprise a Na compound.
Compounds of the invention having the rhombohedral NASICON structure form a framework of MO6 octahedra sharing all of their corners with XO4 tetrahedra (X=Si, P, As, or S), the XO4 tetrahedra sharing all of their corners with octahedra. Pairs of MO6 octahedra have faces bridged by three XO4 tetrahedra to form xe2x80x9clanternxe2x80x9d units aligned parallel to the hexagonal c-axis (the rhombohedral [111] direction), each of these XO4 tetrahedra bridging to two different xe2x80x9clanternxe2x80x9d units. The Li+ or Na+ ions occupy, the interstitial space within the M2(XO4)3 framework. Generally, YxM2(XO4)3 compounds with the rhombohedral NASICON framework may be prepared by solid-state reaction of stoichiometric proportions of the Y, M, and XO4 groups for the desired valence of the M cation. Where Y is Li, the compounds may be prepared indirectly from the Na analog by ion exchange of Li+ for Na+ ions in a molten LiNO3 bath at 300xc2x0 C. For example, rhombohedral LiTi2(PO4)3 may be prepared from intimate mixtures of Li2CO3 or LiOH.H2O, TiO2, and NH4H2PO4H2O calcined in air at 200xc2x0 C. to eliminate H2O and CO2 followed by heating in air for 24 hours near 850xc2x0 C. and a further heating for 24 hours near 950xc2x0 C. However, preparation of Li3Fe2(PO4)3 by a similar solid-state reaction gives the undesired monoclinc framework. To obtain the rhombohedral form, it is necessary to prepare rhombohedral Na3Fe2(PO4)3 by solid-state reaction of NaCO3, Fe{CH2COOH}2 and NH4H2PO4.H2O, for example. The rhombohedral form of Li3Fe2(PO4)3 is then obtained at 300xc2x0 C. by ion exchange of Li+ for Na+ in a bath of molten LiNO3. It will be understood by one of skill in the art that the rhombohedral Na compounds will be useful as cathode materials in rechargeable Na ion batteries.
In another aspect of the invention, the rhombohedral NASICON electrode compounds may have the general formula YxM2(PO4)y(XO4)3xe2x88x92y, where 0 less than yxe2x89xa63, M is a transition-metal atom, Y is Li or Na, and X=Si, As, or S and acts as a counter cation in the rhombohedral NASICON framework structure. In this aspect, the compound comprises a phosphate anion as at least part of an electrode material. In preferred embodiments, the compounds are used in the cathode of a rechargeable battery. Preferred compounds having this general formula include, but are not limited to Li1+xFe2(SO4)2(PO4), where 0xe2x89xa6xxe2x89xa61.
The rhombohedral NASICON compounds described above may typically be prepared by preparing an aqueous solution comprising a lithium compound, an iron compound, a phosphate compound and a sulfate compound, evaporating the solution to obtain dry material and heating the dry material to about 500xc2x0 C. Preferably, the aqueous starting solution comprises FeCl3, (NH4)2SO4, and LiH2PO4.
In a further embodiment, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials have a rhombohedral NASICON structure with the general formula A3xe2x88x92xV2(PO4)3. In these compounds, A may be Li, Na or a combination thereof and 0xe2x89xa6xxe2x89xa62. In preferred embodiments, the compounds are a single-phase rhombohedral NASICON material. Preferred formulas for the rhombohedral NASICON electrode compounds having the general formula A3xe2x88x92V2(PO4)3 include, but are not limited to those having the formula Li2xe2x88x92xNaV2(PO4)3, where 0xe2x89xa6xxe2x89xa62.
The rhombohedral NASICON materials of the general formula A3xe2x88x92xV2(PO4)3 may generally be prepared by the process outlined in FIG. 9. Alternatively, Li2NaV2(PO4)3 may be prepared by a direct solid-state reaction from LiCO3, NaCO3, NH4H2PO4.H2O and V2O3.
In a further aspect, the invention provides a secondary (rechargeable) battery where an electrochemical cell comprises two electrodes and an electrolyte, with or without an electrode separator. The electrodes are generally referred to as the anode and the cathode. The secondary batteries of the invention generally comprise as electrode material, and preferably as cathode material, the compounds described above. More particularly, the batteries of the invention have a cathode comprising the ordered olivine compounds described above or the rhombohedral NASICON compounds described above.